The present invention relates generally to simulating data packet networks, and more particularly to simulating packet delay variation (PDV) in data packet networks for adaptive packet timing recovery stress testing.
An ongoing development in telecommunications is the convergence of voice, video, and data into a common stream. This requires migrating services typically delivered using a circuit network, such as telephony, to a packet based network. However, in a packet based network, synchronization of such services is difficult because there is no longer a precise network clock traceable signal as in a circuit switched network. The network traceable clock is used to recover the service clock of these circuit switched services (e.g., DS1, E1) to ensure error free-transmission. Circuit switched networks rely on the physical layer to transport these network clock signals between network elements to form a timing chain. The accuracy of these physical layer clock signals are typically synchronized to an accuracy of ±4.6 ppm or better. However, in packet networks, the clock signals used at the physical layer do not form a timing chain but are controlled by local free-running oscillators. Further, the accuracy of physical layer transport clock is synchronized to an accuracy of ±100 ppm Therefore, the physical layer clock signals in a packet network are not sufficient to support the error-free transport of circuit switched services over a packet network, commonly called circuit emulation. As a result, other methods must be used to recovery the service clock of circuit emulation services. The method of adaptive timing recovery typically relies on the arrival characteristics of packets as a basis to create a suitable service clock for circuit emulation.
It is well known that adaptive timing methods are sensitive to packet delay variation (PDV) in packet based networks. Accordingly, there has been an effort to take PDV into account when performing timing recovery stress testing for determining performance requirements and testing equipment for use in a packet based network. The current methodology for generating PDV is to inject background traffic with various mixes of packet sizes into a connection-oriented series of packet switches. The packet traffic of interest (PTI) then establishes a path through these switches and experiences delays and delay variation on a switch-by-switch basis. The PDV can be measured as the PTI is received after being transmitted through the packet switches. However, the current methodology is not deterministic or repeatable, since different equipment used to conduct tests can lead to different results. The current methodology cannot control metrics used to model the PDV of real-world networks, such as peak-to-peak variation, packet-to-packet variation, histogram probability density, and statistical specifications, such as mean, mode, and standard deviation. Accordingly, a method for generating PDV that can simulate the PDV of a real world network and provide uniform testing is desirable. This capability is needed for a variety of reasons including the creation of standardized testing methods needed to verify compliance with interface requirements for circuit emulation services.
Furthermore, current methodology for packet timing recovery stress testing is based on Gaussian probability density functions (PDFs). The use of Gaussian distributions favors connection-oriented packet networks where packets flow along the same path and intermediate switches. In connectionless networks, packets traverse different routes, and therefore, will experience a broader range of delays than a simple Gaussian distribution. Accordingly, since Gaussian PDFs infrequently exercise the full delay variation range, they may not be suitable for stress testing adaptive packet timing recovery systems.